U.S. Pat. No. 3,907,926 and French Pat. No. 2,194,741 disclose blends of polybutylene terephthalate with thermoplastic copolyester which consists essentially of a multiplicity of recurring long chain ester units and short chain ester units joined head to tail through ester linkages. Long chain ester units of the copolyester polymers are disclosed as being represented by the structure: ##STR1## and said short chain ester units are represented by the structure: ##STR2## wherein:
G is a divalent radical remaining after removal of terminal hydroxyl groups from poly(alkylene oxide) glycols having a carbon-to-oxygen ratio of about 2.0-4.3 and molecular weight between about 400 to 6000;
R is a divalent radical remaining after removal of carboxyl groups from a dicarboxylic acid having a molecular weight less than about 300; and
D is a divalent radical remaining after removal of hydroxyl groups from a low molecular weight diol having a molecular weight less than about 250.
Preferred copolyester for use in such blends is prepared from dimethyl terephthalate, 1,4-butanediol and polytetramethylene oxide glycol. A wide variety of other glycols, diols and acids suitable for use in making copolyester for blending with polybutylene terephthalate are also disclosed. The use of 1,4-butenediol in such copolyester is not, however, disclosed.
Various improvements in mechanical properties such as tear strength, tensile strength, flex modulus, low temperature impact strength and flexibility are alleged for the blends taught by the U.S. Pat. No. 3,907,926 and data is presented establishing improvement in physical properties for a preferred blend of polybutylene terephthalate with copolyester made from polytetramethylene ether glycol, 1,4-butanediol and dimethyl terephthalate as compared to the random copolymer itself.
U.S. Pat. No. 4,355,155 discloses segmented thermoplastic copolyester elastomers which are generally similar to preferred embodiments of the copolyester component of the U.S. Pat. No. 3,907,926 except that in the copolyester elastomer of the 4,355,155 patent the short chain ester units are derived from phthalic acis and a mixture of 1,4-butanediol and 1,4-butenediol.
While blends as taught by the above mentioned U.S. Pat. No. 3,907,926 have certain advantages, melt rheology shows that these blends form less stable melts than the random copolymers. Poor melt stability can result in processing difficulties. In fact, U.S. Pat. No. 3,907,926 requires that the polybutylene terephthalate used to form the blends have a melt index no greater than 50, and preferably between 1 and 25, g/10 min at 250.degree. C., in order to avoid adversely affecting processing characteristics.